An aneurysm is a circumscribed dilation of a blood vessel, or cardiac chamber, in direct communication with its respective lumen, usually resulting from an acquired or congenital weakness of the wall of the blood vessel, or chamber. An aneurysm occurs when a part of the artery, other blood vessel, or cardiac chamber, swells, either due to damage to the wall or a weakness in the wall. As blood pressure builds up with each heartbeat, the wall balloons out at its weakest point, forming an aneurysm bubble. The bubble tends to fill with blood, as blood flowing through the lumen is partly diverted through the opening into the aneurysm. As the aneurysm grows, in addition to causing pain and other complications, a risk of rupture of the bubble increases. Rupture of an aneurysm in a carotid or vertebral artery in the neck, or in a branch blood vessel extending from the carotid or vertebral artery into the brain can cause a hemorrhage or stroke, which can be at best severely debilitating, and at worst deadly. For perspective, aneurysmal subarachnoid hemorrhage (SAH) occurs about 30,000 times annually in the United States. Of these, between ⅓ and ½ of those so afflicted will not survive the trip to the hospital.
FIGS. 1 and 2 depict various configurations of aneurysms. In each drawing, a generally tubular blood vessel 10 comprises an inner peripheral wall 12, and an outer peripheral wall 14. The inner peripheral wall 12 defines a lumen 16 through which blood 17 flows. A weak point in the wall 12 has an opening, called a neck, 18. A portion of the blood flow 17 is diverted from the lumen 16 through the neck 18 to form and maintain a saccular aneurysm 20. A saccular aneurysm 20 is shown in FIG. 1. FIG. 2 shows a fusiform aneurysm 20′. If the arterial wall weakness is focal, i.e., it does not extend all the way around the artery, the aneurysm is most likely to be saccular. If the weakness is circumferential, it will more likely be fusiform. In the saccular aneurysm 20, its communication with the lumen 16 is though the aneurysm neck 18. The fusiform aneurysm 20′ by definition does not have a neck.
FIGS. 3A-3B depict a conventional attempt to treat aneurysms. FIG. 3A depicts a wire stent 30, installed in a blood vessel 10 with a fusiform aneurysm bubble 20′. Referring to FIG. 3B, a raised strut portion 32 of the wire stent 30 depicted in FIG. 3A is provided against the blood vessel inner peripheral wall 12 immediately upstream of the aneurysm neck 18. It is intended that an increase in velocity of the blood flow 17 over the raised strut portion 32 will create a pressure drop (dP) at a trailing edge thereof to cause blood 17 in the saccular aneurysm bubble 20 to flow back into the lumen 16 of the blood vessel 10.
FIG. 4 explains why this conventional attempt to treat aneurysms is sometimes ineffective, even harmful. At the trailing edge of the raised strut portion 32, as in the case of a trailing edge of an airplane, the flow of fluid (in this case blood 17) flows across the apex of the wing and down past the trailing edge. The configuration and positioning of the strut 32 in FIG. 3B, however, rather than creating a pressure drop (dP), drawing blood 17 out of the saccular aneurysm 20 and into the lumen 16, instead directs more blood flow 17 at the trailing edge into the saccular aneurysm 20, thereby achieving an effect opposite of that which was intended. In fact, it can have the effect of directing high-velocity blood to strike the inflow zone of the aneurysm, a fragile, delicate part.